This particular ruler uses a type of projection called Linear Perspective ( Isometric and Fisheye projections are also available as different rulers). It generates a set of guide lines that connect your current pen position to fixed vanishing points. Your lines then snap to these guides when you draw, helping you quickly construct perspective-correct objects in your scene. You have complete control over the camera setup, and can easily rotate the vanishing lines in 3D to draw objects in any orientation.
If you are working with an existing image with an unknown perspective setup, and would like to add perspective-correct elements to it, you may want to use the Vanishing Lines ruler instead. That ruler doesn't require you to know or setup any camera parameters. Instead, you can set the vanishing points directly, or have them setup automatically by lining up control lines with parallel lines from your scene. And if you're using a recent enough version, you can then easily have the Linear Perspective parameters automatically set up to match the Vanishing Lines ruler's perspective.